Fractional separation of silver halide grains



June 20, 1967 R, G. AUDRAN ETAL 3,326,641

FRACTIONAL SEPARATION OF SILVER HALIDE GRAINS Filed Feb. 12, 1964 2Sheets-Sheet 1 ROGER GASTON AUDRAN MICHEL GIBER T ANDRE HENRI HOLLANDINVENTORS ATTORNEYS June 1967 R. G. AUDRAN ETAL' 3,3 ,6

FRACTIONAL SEPARATION OF SILVER HALIDE GRAINS Filed Feb. 12, 1964 2Sheets-Sheet 2 MICHEL G'IBE/PT ANDRE HENRI ROLLA/VD M m U A N m m 6 m mR INVENTO ATTORNEYS United States Patent 3,326,641 FRACTIONAL SEPARATIONOF SILVER HALIDE GRAINS Roger G. Audran, Vitry-sur-Seine, Michel Gibert,Couilly-Pont-aux-Dames, and Andre H. Rolland, Rosny-sous- Bois, France,assignors to Eastman Kodak Company, Rochester, N.Y., a corporation ofNew Jersey Filed Feb. 12, 1964, Ser. No. 344,361 3 Claims. (Cl. 23-305)This invention relates to the manufacture of silver halide photographicemulsions which involves classification of the grain size by means ofhydrocyclones.

In the course of the conventional preparation of lightsensitiveemulsions, it may be desirable to sort-out, according to size, theripened silver halide crystals in suspension in a colloidal medium, inorder, for instance, to separate the large highly sensitive grainsand/or the fine insensitive grains. 1

More generally, it is desirable to be able to prepare, as wanted,homogeneous categories of physically ripened grains whose subsequentapplication would be more flexible and efficient, irrespective whetherthe sensitization or coating is contemplated in the form of a mixture oras superposed layers.

The conventional separation processes (sedimentation or mechanicalcentrifuging) hitherto used for the sorting out of silver halidecrystals in the process of formation result in a more or less broadstatistical distribution pattern of sizes, narrowing down of which is inmost cases indicated; sedimentation requires keeping the dispersion inthe liquid phase for long periods of time, thereby increasing foggingpropensity, whereas mechanical centrifuging of the colloidal suspensionof silver halide crys tals whose calibration is not very precise causesgrain clumping and deposits, as well as the crushing of grains againstthe walls of the device, factors that are all prejudicial to the qualityof the light-sensitive emulsion, especially in view of the fact that allof these methods tend to fog the silver halide grains. Furthermore, in acontinuous manufacturing process, the slow rate of growth of thecrystals and the durations of ripening necessitate very long circuits.

The process according to the invention consists in insetting in themanufacturing circuit of the light-sensitive emulsion a ripening zone ofsuitable volume in which, by application of a field of centrifugalforce, the silver halide crystals are maintained in continuouscirculation, and from which they are continuously extracted andevacuated as they grow to a set size, the crystals having achieved thesaid size being immediately removed from the centrifugal force field.

For the purpose of production by the continuous process, those of saidcrystals that have not achieved the desired size are, if necessary,reintroduced into the circuit comprising the said ripening zone or aplurality of series-connected ripening zones, in order there to continuetheir growth.

The zones of application of the field of centrifugal force may,according to one embodiment, be linked by means of a cascade-connectionfor the purpose of separating and classifying crystals of various sizes,with the number of categories depending on the number of zones.

For the application of the process according to the invention, the fieldof centrifugal force to which the silver halide crystals in suspensionare subjected for the purpose 3,326,641 Patented June 20, 1967 ICC ofseparating them into granulometrically different categories withoutcombining the crystals in the form of clumpings or deposits, and withoutsubjecting them to friction and crushing, is created by a hydrocycloneor a set of hydrocyclones mounted on a closed circuit in which asuitable device provides for continuous circulation of the suspension.

In the annexed drawing, given merely by way of example;

FIG. 1 is a partial schematic representation of a device according tothe invention for the preparation of lightsensitive emulsions, showingthe ripening zone and the generator of the field of centrifugal force.

FIG. 2 is a schematic representation of the same part of another device,according to another application of the process of the invention,wherein the hydrocyclones are shown in cascading-connection, for theseparation of grains of one and the same size.

FIG. 3 is a schematic representation of a cascadingconnection of threehydrocyclones for the separation of grains of three differentcategories.

FIG. 4 is a schematic representation of a hydrocyclone on which areindicated the symbols of the various geometric parameters particularlysuitable for the treatment of light-sensitive emulsions.

FIG. 5 is a diagram of the treatment equipment according to theinvention.

One-half of FIG. 6 is a side-view and one-half is a cross-sectional viewof a hydrocyclone according to the invention.

FIG. 7 represents a battery of eight hydrocyclones.

According to one embodiment shown in FIG. 1, the colloidal dispersion ofsilver halide crystals, whose viscosity is adjusted to a suitable level,is formed in a tank 2 whose volume varies with the equipment dischargeand the dispersion characteristics and into which the custom'aryreagents are introduced via the pipes 1, 1'. The

colloidal dispersion is discharged under pressure by means of a pump 3into a pipe 4 extending into the generator of dispersion formed by theintroduction of reagents via the pipes 1, 1'. It can thus be seen thatthis arrangement of the equipment according to one embodiment of theinvention makes it possible, in particular, to produce continuously alight-sensitive emulsion by means of a very short treatment circuit thatnevertheless permits the ripening of the silver halide crystals.

According to the embodiment of FIG. 2 the pipe 7 for the removal of thecrystals of a size below that desired to be achieved feeds a secondgenerator 5' which, in turn, feeds another generator 5". The equipmentset upaccording to this arrangement works like a radioelectric band-passfilter: the finest silver halide crystals unsuited for'the manufactureof a light-sensitive emulsion are evacuated from the generator 5" viathe pipe 7", whereas all crystals of a size greater than that of the"grains removed via the pipe 7", and smaller than that wanted size) arereintroduced via the pipes 6' and 6" into the tank 2 where they continuetheir growth mingled with the fresh dispersion initially introduced viathe pipe 1. It is quite obvious that one may arrange in groups anynumber of generators according to this type of connection.

FIG. 3 represents a cascading connection of three generators 5, 5' and 5of different characteristics making it possible, respectively, to selectgrains of decreasing sizes. The discharge pipes 6 and 6 of the last twogenerators do not reintroduce the selected grains into the tank 2, notshown. These silver halide crystals of determined and different sizesthat are thus separated and retained are used for the manufacture,either by themselves or in mixtures according to well-defined ratios oflight-sensitive emulsions having different characteristics. Any numberof generators may be arranged in groups in that manner.

FIG. 4 which represents in a schematic cross-section a generator of acentrifugal force field, used according to the process that is theobject of the invention, indicates the various geometric parametersdetermining the proper operation of the generator for the sorting out ofthe silver halide crystals. The generator utilized according to theinvention is a hydrocyclone, a known device, used for the separation ofa mixture of two phases at least one of which is liquid.

The excellent results achieved in the preparation of light-sensitiveemulsions through the use of hydrocyclones are unexpected and novel.Indeed, as was recalled above, the utilization of a field of centrifugalforce generated by mechanical centrifugal machines for the sorting outof silver halide crystals causes grain clumping and deposits, as well aspossible fogging through the crushing of grains against the walls. Onemight have expected to find also the same shortcomings by usinghydrocyclones, whereas, on the very contrary, the practice showed thatthe aforementioned shortcomings disappear.

The suitably diluted liquid dispersion is at a given pressure introducedinto the hydrocyclone where it is for an extremely short period of timesubjected to the centrifugal force providing for the separation of thegrains into two granulometric categories lying on either side of apreset size. This shortened treatment period eliminates the alterationlikely to result from extended treatments,

as occurs in the sorting-out process by sedimentation. On the otherhand, no silver halide material is subjected to the centrifugal forceafter the separation occurred, contrary to the procedure in mechanicalcentrifugal machines. The centripetal discharge of the dispersionmaintains the grains in the dispersed state and does not promote theformation of deposits and grain clumping, usual sources of fogging;furthermore, it eliminates their crushing against the walls, as a resultof which fogging may be produced with emulsions that are vulnerable andof high sensitivity.

The following, non-limiting, example illustrates an application of themanufacturing process according to the invention.

Example A negative, high-speed, silver bromoiodide emulsion containing12 grams gelatin per mole of silver is diluted to about 5 kg. per moleof silver. Under these conditions, 51 moles of emulsion under a pressureof 150 p.s.i. are introduced into a hydrocyclone whose cylindricalchamber has a diameterof 5 mm. This hydrocyclone is a part of theinstallation schematically shown in FIG. 5, in which each cyclone worksindependently.

Following separation within the installation, the emulsion isconcentrated by coagulation, then the gelatin is added to produce about80 grams per mole of silver and the overall weight is adjusted to 1.5kg. per mole of silver.

The emulsions prepared in this manner from the various fractionalamounts discharged by the different hydrocyclones are coated on asupport to allow determining of the sensitometric characteristics in thecustomary manner.

These results are recorded in the table below where the differentemulsions refer to the number of the hydrocyclone from which theyoriginated followed by the letter S (suprieur=upper) or I(infrieur=l0wer) depending whether they originate from the materialfraction discharged by the upper or lower aperture, respectively.

Total Radius of Average percent- Percent largest area of Emulsion age ofmole grain grains 7 Fog emulsion Ag I (in ,1) (in ,1

collected An emulsion prepared from grains sorted out according toconventional processes, e.g., by sedimentation, which, as indicated byF. F. Renwick in the British Photographic Journal of Aug. 1924 (p. 362)creates great problems, for the emulsion has to be kept liquid for longperiods of time while making it preserve its photographic qualitiesduring the sedimentation, produces, after treatment, fogs of 0.10 in thecase of the emulsion prepared with grains of the upper layer and of 0.22in the case of the emulsion prepared with grains of the lower layer, fogvalues considerably greater than those achieved by the process accordingto the invention. Indeed, the above table indicates that the fog densityis, with emulsions treated according to the invention, practicallyidentical to that produced by the identical emulsion that has not beentreated.

The use of hydrocyclones offers, besides, great advantages with regardto plan-t maintenance, for one uses static devices that do not compriseany moving part, whose wear and clogging up are practically zeroprovided the dispersion has been properly filtered.

Practice proved that all the geometrical parameters determining theproperties of the hydrocyclone used for the preparation ofphotosensitive emulsions can be brought in relationship with theinternal diameter Dc of the cylindrical chamber of elevation Hc. Do andL designate the diameter and the length of the central pipe forseparation of the ascending and descending, eddying liquid fluxes,herebelow referred to merely as pipe; and, respectively, Di the diameterof the input tube 4a, Ds that of the lateral aperture 7a of theevacuation chamber 14, Du the diameter of the discharge aperture 6a, and0 the angle of the cone of the lower section.

In practice, these parameters must suitably satisfy the followingrelationships:

Ds D0 Do 10 mm.

made of stainless steel whose grade is suitable for the emulsionstreated; the pipe bearing liner 10 is made of plastic capable ofprecision machining and providing tightness of the assembled unit. Theinput tube 4a and the lateral aperture 7a of the evacuation chamber 14are situated on the same outer generatrix of the cylinder; the inputtube 4a extends tangentially into the interior of the cylindricalsection of the body 8 whereas the lateral aperture 7a is piercedradially. The lower planes 9' and 10' of the plug 9 and the liner 10 aretangential to the pipes 7a and 4a, respectively. The practice provedthat the use of small anglesof 0, below 0.157 rd., transforms thehydrocyclone into a separator, the hydrocyclone will then separate theliquid phase from the particles in suspension, instead of sorting themout.

In the case of industrial applications it is often necessary to useequipment having a large output. To this end, one may at each separationstage use batteries of hydrocyclones parallel-supplied as shown in FIG.7. Each one of the hydrocyclones, a total of eight in the example shown,presses firmly against the feeder pipes 4 and the upper discharge pipe 7by means of an inserted seal, not shown, between each aperture of thehydrocyclone housings and a corresponding aperture of said pipes. Thecone of the body 8 of each hydrocyclone ends in a spherical calotte 11(FIG. to be fitted into a tapered hole 12 of the discharge pipe 6, thusgiving the housing 8 a certain degree of angular play indispensable forthe flattening of the seals of the pipes 4 and 7. An adjustable pressurescrew 13 makes it possible to provide for tight sealing among thehydrocyclone components 8, 9 and and between the housing 8 and thedischarge pipe 6.

Assembled in this manner, this hydrocyclone battery supplied under apressure of 686 k. Pa is capable of processing 120 cubic decimeter/hr.of colloidal silver halide dispersion.

It is well understood that the invention is not limited to the variousembodiments described and illustrated which were selected as examplesonly.

We claim:

1. A method of treating an aqueous dispersion of silver halide grains inthe process of ripening which comprises, introducing reagants into afirst vessel to form an aqueous dispersion of silver halide grains,running the dispersion under pressure into a hydrocyclone separatorwhereby the dispersion is subjected to centrifugal force in a zone ofthe separator, wherein silver halide crystals are further ripened to aselected large size withdrawing the larger silver halide grains from theseparator as they are formed, withdrawing and returning the smallundeveloped grains to said reagent vessel for mingling with the freshamount of dispersion formed by the introduction of reagents therein.

2. The method of claim 1 in which the said dispersion is introducedunder pressure through a series of hydrocyclone separators in cascadingconnection whereby the dispersion is subjected to centrifugal force in azone of each separator, withdrawing the large grains from the systemonly from the bottom of the first separator as they are formed,returning small grains from the separator to said first reagent vessel.

3. The method of claim 1 in which the said dispersion is introduced intoa series of hydrocyclone separators in cascading connection withdrawingfrom the system grains from the bottom of each of said hydrocycolneseparators whereby the silver halide grains are separated into differentcategories of grains of a given size.

References Cited UNITED STATES PATENTS 2,620,264 12/1952 Watson 23-3052,707,669 5/1955 Houston 23273 2,819,154 1/1958 Frejacques 23-2733,202,487 8/1965 Domning 23-295 NORMAN, YUDKOFF, Primary Examiner. G.HINES, Assistant Examiner.

1. A METHOD OF TREATINGD AN AQUEOUS DISPERSION OF SILVER HALIDE GRAINSIN THE PROCESS OF RIPENING WHICH COMPRISES, INTRODUCING REAGENTS INTO AFIRST VESSEL TO FORM AN AQUEOUS DISPERSION OF SILVER HALIDE GRAINS,RUNNING THE DISPERSION UNDER PRESSURE INTO A HYDROCYCLONE SEPARATORWHEREBY THE DISPERSION IS SUBJECTED TO CENTRIFUGAL FORCE IN A ZONE OFTHE SEPARATOR, WHEREIN SILVER HALIDE CRYSTALS ARE FURTHER RIPENED TO ASELECTED LARGE SIZE WITHDRAWING THE LARGER SILVER HALIDE GRAINSD FROMTHE SEPARATOR AS THEY ARE FORMED, WITHDRAWING AND RETURNING THE SMALLUNDEVELOPED GRAINS TO SAID REAGENT VESSEL FOR MINGLING WITH THE FRESHAMOUNT OF DISPERSION FORMED BY THE INTRODUCTION OF REAGENTS THEREIN.